Enormous efforts are being directed to research and development of light-weight high-efficiency flat-panel displays for use as computer screens or television screens.
One reason for this is that the conventional CRT now in general use as the display device is bulky, heavy, and power-consumptive despite its high luminance and good color reproduction.
Among the light-weight high-efficiency flat-panel displays which have recently been commercialized is, for example, a liquid-crystal display of active matrix drive type. Unfortunately, it has a narrow viewing angle and needs back light (which consumes a large amount of electric power) for use in a dark place because it does not emit light by itself. Moreover, it cannot quickly respond to high-definition high-speed video signals to be put to practical use in near future. Another problem is high production cost and technical difficulties involved in production of large-sized display.
A possible substitute for liquid-crystal display is a display with light-emitting diodes. However, it also has a problem with high production cost and technical difficulties in forming light-emitting diodes in matrix structure on a single substrate. It is not an inexpensive display to replace CTRs in near future.
There has recently appeared a flat panel display which is expected to solve the above-mentioned problems. It is an organic electroluminescent element (organic EL element) made with an organic luminescent material. It is expected that a specific organic compound as a luminescent material will help realize a flat display panel which emits light by itself, responds quickly, and ensures good visibility free from viewing angles.
The organic EL element is composed of a transparent anode and a metal cathode and an organic thin film interposed between the anode and the cathode which contains a luminescent material which emits light upon injection of electric current. That of single hetero structure, which was developed by C. W. Tang and S. A. VanSlyke, is reported in Applied Physics Letters, vol. 51, No. 12, pp. 913 to 915 (1987). It has an organic thin film of double-layer structure, which is composed of a thin film of hole transporting material and a thin film of electron transporting material, so that luminescence takes place upon recombination of holes and electrons injected into the organic thin film from each electrode.
The organic EL element of this structure permits either the hole transfer material or the electron transfer material to function as a luminescent material. Luminescence takes place in the wavelength region corresponding to the energy gap between the ground state and the excited state which are assumed by the luminescent material. The double-layer structure brings about a considerable reduction in driving voltage and a great improvement in luminescence efficiency.
At a later time, an organic EL element of double hetero structure was reported by C. Adachi, S. Tokita, T. Tsutsui, and S. Saito in Japanese Journal of Applied Physics, vol. 27, No. 2, pp. L269 to L271 (1988). The structure is composed of three layers of hole transporting material, luminescent material, and electron transporting material. Further, another organic EL element was reported by C. W. Tang, S. A. VanSlyke, and C. H. Chen in Journal of Applied Physics, vol. 65, No. 9, pp. 3610 to 3616 (1989). The structure has a luminescent material contained in the electron transporting material. These researches have proved the feasibility of intensive luminescence at a low voltage. Active research and development works in this field are going on.
The fact that there exists a large variety of organic compounds used as luminescent materials suggests that it would be possible, at least theoretically, to produce light of any desired color if their molecular structure is properly modified. In other words, properly designed organic compounds would give three pure colors (red, green, and blue) necessary for full color display more easily than inorganic compounds used for thin-film EL elements.
It is reported in non-patent literature (1) listed below that the emission of red color is possible with an electron transporting material which is tris(8-quinolyl)aluminum (Alq3 for short hereinafter) doped with 4-dicyanomethylene-6-(p-dimethylaminostyryl)-2-methyl-4H-pyran (DCM for short hereinafter).
It is also reported in non-patent literature (2) listed below that BSB-BCN gives a luminescence of 1000 cd/m2 or above.
It is proposed in patent literature (1) listed below that a specific styryl compound be used as the electroluminescent material.                Non-patent literature (1): Chem. Funct. Dyes, Proc. Int. Symp., 2nd. p.536 (1993)        Non-patent literature (2): T. Tsutsui, D. U. Kim, Inorganic and Organic Electroluminescence Conference (1996, Berlin)        Patent literature (1): Japanese Patent Laid-open No. Hei 7-188649 (Claim, from p. 5, right column, line 8, to p. 22, right column, line 5, and FIGS. 1 to 3)        
Despite these prior art technologies, the actual organic electroluminescent element still have problems to be solved. Difficulties are involved in development of an element capable of stably emitting red light with a high luminance. The electron transporting material (DCM-doped Alq3) reported in the non-patent literature (1) above does not exhibit satisfactory luminance and reliability required of the display material.
The BSB-BCN reported in the non-patent literature (2) above gives a luminance of 1000 cd/m2 or above but does not give a complete red chromaticity necessary for full color display.
There is a need for development of an electroluminescent element capable of stably emitting red light with a high purity and a high luminance.
The specific styryl compound, which proposed for use as the organic electroluminescent material in the patent literature (1) above, is intended only for blue light emission. It is not intended for emission of light with wavelengths of red or other colors.
It is an object of the present invention to provide an organic electroluminescent element and a light-emitting device or display device incorporated therewith. The organic electroluminescent element is formed from a compound which has a high fluorescence yield and a good thermal stability. Moreover, it stably and selectively emits light of red and other colors (in the region of comparatively long wavelengths) with a high color purity and a high luminance.